The present invention relates to manufacture of integrated circuit devices using microlithography techniques. In particular, the present invention is directed to a mask and method for creating isolated features that have similar physical characteristics to similar features that are not isolated.
Very large scale integrated circuit devices typically are manufactured on a substrate, such as a silicon wafer, by a sequence of material additions, such as low pressure chemical vapor depositions, sputtering operations, among others; material removals, such as wet etches, reactive ion etches, among others; and material modifications, such as oxidations, ion implants, among others. Typically, these physical and chemical operations interact with the entire substrate. For example, if a substrate is placed into an acid bath, the entire surface of the substrate will be etched away. In order to build very small electrically active devices on a substrate, the impact of these operations has to be confined to small, well-defined, regions.
Lithography in the context of VLSI manufacturing includes the process of patterning openings in photosensitive polymers, sometimes referred to as xe2x80x9cphotoresistsxe2x80x9d or xe2x80x9cresistsxe2x80x9d, which define small areas in which substrate material is modified by a specific operation in a sequence of processing steps.
The radiation preferably causes desired photochemical reactions to occur within the photoresist. Preferably, the photochemical reactions alter the solubility characteristics of the photoresist, thereby allowing removal of certain portions of the photoresist. Photoresists can be negative photoresist or positive photoresist materials.
A negative photoresist material is one which is capable of polymerizing and being rendered insoluble upon exposure to radiation. Accordingly, when employing a negative photoresist material, the photoresist is selectively exposed to radiation, causing polymerization to occur above those regions of the substrate which are intended to be protected during a subsequent operation. The unexposed portions of the photoresist are removed by a solvent which is inert to the polymerized portion of the photoresist. Such a solvent may be an aqueous solvent solution.
Positive photoresist material is a material that, upon exposure to radiation, is capable of being rendered soluble in a solvent in which the unexposed resist is not soluble. Accordingly, when applying a positive photoresist material the photoresist is selectively exposed to radiation, causing the reaction to occur above those portions of the substrate which are not intended to be protected during the subsequent processing period. The exposed portions of the photoresist are removed by a solvent which is not capable of dissolving the exposed portion of the resist. Such a solvent may be an aqueous solvent solution.
Selectively removing certain parts of the photoresist allows for the protection of certain areas of the substrate while exposing other areas. The remaining portions of the photoresist may be used as a mask or stencil for processing the underlying substrate. For example, the openings in the mask may allow diffusion of desired impurities through the openings into the semiconductor substrate. Other processes are known for forming devices on a substrate.
The manufacturing of VLSI chips typically involves the repeated patterning of photoresists, followed by etch, implant, deposition, or other operation, and ending with the removal of the exposed photore!sist to make way for the new photoresist to be applied for another iteration of this process sequence.
Devices such as those described above, may be formed by introduction of a suitable impurity into a wafer of a semiconductor to form suitably doped regions therein. In order to provide distinct P or N regions, which are necessary for the proper operation of the device, introduction of impurities should occur through only a limited portion of the substrate. Usually, this is accomplished by masking the substrate with a diffusion resistant material, which is formed into a protective mask to prevent diffusion through selected areas of the substrate.
Basic lithography systems typically include a source of light, typically not visible light, a stencil or photomask including a pattern to be transferred to a substrate, a collection of lenses, and a means for aligning existing patterns on the substrate with patterns on the mask or stencil.
Conventional photomasks typically consist of chromium patterns on a quartz plate, allowing light to pass wherever the chromium has been removed from the mask. Light of a specific wavelength is projected through the mask onto the photoresist coated substrate, exposing the photoresist wherever chromium has been removed from the mask permitting light to pass through the mask. Exposing the resist to light of the appropriate wavelength causes modifications in the molecular structure of the resist polymers, which permits the use of developer to dissolve and remove the resist in the exposed areas. Resists that act as just described are known as xe2x80x9cpositivexe2x80x9d resists. On the other hand, negative resist systems permit only unexposed areas to be removed by the developer.
Photomasks, when illuminated, can be pictured as an array of individual, infinitely small light sources that can be either turned on, such as areas not covered by chromium or other material, or turned off, such as areas covered by chrome or other material. If the amplitude of the electric field vector that describes the light radiated by these individual light sources is mapped across a cross-section of the mask, a step function will be plotted reflecting the two possible states that each point of the mask can be found, either light on or light off.
Conventional photomasks are commonly referred to as xe2x80x9cchrome on glassxe2x80x9d (COG) binary masks, due to the binary nature of the image amplitude. The perfectly square step function of binary masks actually exists only in scalar theory and typically only in the level of the exact mask plane. Any distance away from the mask, such as at the substrate plane, diffraction will cause images to exhibit a finite image slope. At small dimensions, that is, when the size and spacing of the images to be printed are small relative to the wavelength and inverse of the numerical aperture, the electric field vectors of adjacent images will interact and add constructively.
Therefore, not only is diffraction a phenomenon that must be addressed when dealing with very small images, interference must also be addressed. The resulting light intensity curve between features is not completely dark, as a result of the diffraction and interference phenomenon. Rather, the light intensity curve exhibits significant amounts of light intensity created by the interaction of adjacent features.
The resolution of an exposure system is limited by the contrast of the projected image, that is, the intensity difference between adjacent light and dark features. An increase in the light intensity in nominally dark regions will eventually cause adjacent features to print as one combined structure rather than as discrete images.
In an effort to increase the capability of electronic devices, the number of circuit features included on, for example, a semiconductor chip, has greatly increased. When using a process such as that described above for forming devices on, for instance, a semiconductor substrate, increasing the capability and, therefore, the number of devices on a substrate requires reducing the size of the devices or circuit features. One way in which the size of the circuit features created on the substrate has been reduced is to employ mask structures having smaller openings.
Such smaller openings treat smaller portions of the substrate, thereby creating smaller structures in the photoresist. In order to produce smaller structures in the photoresist, shorter wavelength ultraviolet radiation is also used in conjunction with the mask to image the photoresist. Such shorter wavelengths of radiation have also been particularly effective at curing or hardening photoresist materials used in fabricating the devices.
The increasingly small and densely packed devices being formed on semiconductor substrates increases the effects of diffraction, for example. As discussed above, radiation passing through a mask in semiconductor device manufacture processes behaves as it does in any other context. Accordingly, as a result of diffraction of the radiation, the radiation may expose areas of the substrate not directly in line with the transparent area of the mask that the radiation is passing through. Diffraction of radiation may result in a feature of an intended size being formed of different sizes, as a result of whether the feature is formed in a group of other features or in isolation.
Aspects of the present invention provide a microlithography mask for producing equal size features in a substrate. A first region of the mask is for exposing a first portion of the substrate corresponding to a first feature that is to be performed on a substrate. The mask also includes at least one compensating region in the vicinity of the first region for partial exposing the first feature and it also exposing a second portion of the substrate corresponding to a second feature, wherein the second feature is to be removed from the substrate.
Aspects of the present invention also provide a method of forming an isolated image segment having physical characteristics of an optical proximity affected segment in a semiconductor mask. A portion of a first photoresist layer on the substrate corresponding to a first feature that is to be formed in the first photoresist layer is exposed. At least one compensating portion of the first photoresist layer adjacent the first region is exposed such that exposing the compensating portion at least partially exposes the first feature. The at least one compensating feature is subsequently removed.
Still other aspects, objects, and advantages of the present invention will become readily apparent by those skilled in the art from the following detailed description, wherein it is shown and described only the preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive.